1. Field of the Invention
The present invention relates to an electric contact More specifically, the invention relates to electric contacts used as plug-in contacts, for example in plug-in connectors, in automobiles and telecommunications applications.
2. Description of Related Art
Today, it is a requirement that plug-in connectors used in the automotive industry be suited for ambient temperatures of up to 150° Celsius and that their spring properties should not decrease over the usual service life of automobiles to an extent that would impair their contact-making reliability. Known electrical contacts for such applications consist of a main body, made from a copper-based alloy that provided the required electric conductivity and spring properties, and from a hard gold layer that, which is applied onto the main body by galvanic deposition and which consists of gold with a cobalt content of less that 1 percent by weight. Further it has been known to provide on the main body, as a contact layer, a silver layer instead of a hard gold layer. Frequently, one also uses contact layers consisting of tin, which are applied upon the main body by tinning. Given the marginal conditions required hereto for, it has been possible in this way to achieve a sufficient degree of wear resistance of the electric contacts and a sufficiently low contact resistance. This is, however, no longer true for plug-in contacts, which are required, for example according to US Car Specifications, to meet increased temperature demands of up to 200° Celsius under changing temperature conditions over the envisaged service life. These stricter demands result from the fact that an ever increasing number of engine functions are to be monitored and controlled electrically or electronically, for which purpose the use of the electronic system and, thus, of plug-in contacts, is required at the very location on the engine or in the exhaust system.
The gold-cobalt contact layers used heretofore are not suited for such increased temperature demands because cobalt will segregate from the alloy at temperatures above 150° Celsius with the result that the cobalt will then be able to oxidise which in turn will increase the contact resistance. Tinned contacts cannot be used at temperatures of 200° Celsius, either, because that temperature is near the melting point of tin, namely 232° Celsius, and the tin will start to soften and to creep. The accelerated diffusion of Sn in Cu, and vice versa, very rapidly leads to the formation of intermetallic phases which oxidise and lead to high contact resistance. In the case of Ag coatings, irreversible softening occurs at temperatures of approximately 160° Celsius and over.
In telecommunication applications very high insertion cycles—frequently of up to 10,000—are required. Today, these demands are met by plug-in contacts comprising a PdNi or PdCo coating as contact layer. However, the strongly risen Pd price has made such coatings very expensive.